PATH-ANALYSIS OF NATURAL-SELECTION ON PLANT CHEMISTRY - THE XYLEM RESIN OF PONDEROSA PINE

We analyzed the pattern of correlations among fitness components, herb ivory, and resin characteristics in a natural all-aged stand of ponder osa pine, to infer the strength and mechanism of natural selection on plant chemistry. Male and female cone production were monitored yearly for 15 years, and levels of herbivory for 9 years in 165 trees. Resin flow rate and monoterpene composition were determined for these same trees. Multiple regression of fitness components on resin characterist ics showed significant associations consistent with directional select ion for increased resin flow rates and increased proportions of alpha- and beta-pinene, myrcene and terpinolene. However, negative correlati ons among monoterpene fractions of the resin constrained the overall s election. Selective herbivory by aphids approached statistical signifi cance and monoterpenes showed some (non-significant) effect as deterre nts against deer browse. Resin characteristics were not correlated wit h attack by cone insects or porcupines. However, the association betwe en resin characteristics and fitness is significantly different from t hat predicted by the path coefficients involving herbivores. Therefore the hypothesis that these herbivores mediate selection on the resin i s not supported by the observed pattern of correlations among resin ch aracteristics, herbivory, growth and fecundity. In this population, mo st of the association between resin characteristics and fitness appear s to be mediated by some other factor independent of attack by herbivo re species present.